Porosity is one of the important parameters in petrophysical evaluations of petroleum reservoirs, and is defined as the interconnected pore volume or void space in a rock that contributes to fluid flow or permeability in a reservoir. Porosity describes the pore volume that exists in a geological formation and is important to petroleum engineers, because pore networks may contain economically-valuable hydrocarbons, and is a useful measure in estimating potential petroleum reserves. Types of porosity measurements include effective porosity, which excludes isolated pores and pore volume occupied by water adsorbed on clay minerals or other grains, and total porosity, which is the total void space in the rock whether or not it contributes to fluid flow. Porosity may also be used to determine permeability of the formation, which may provide estimates of production rates, the location of completion and perforation intervals, and potential injection behavior and patterns.
A number of methods exist for measuring porosity, including calculating porosity from bulk density and grain density on core samples taken from a selected formation. Analytical measurement techniques used to determine porosity may include gas saturation with pressure-volume control, liquid saturation with weighting, and petrographic analysis of thin sections. However, such methods of formation evaluation are disadvantageous in that they are relatively time-consuming, complex, may require intact core samples, and are better suited for laboratory analysis rather than use in the field.